A certain degree of asymmetry always exists even in a newly manufactured machine. If the percentage of asymmetry exceeds 10% of the nominal air gap length, the machine is said to be eccentric. Eccentricity results in excessive noise, vibration, higher torque ripple, increased electromagnetic stress, increased temperature and in the worst case causes a stator rotor rub. Due to such detrimental effects, monitoring the motor for eccentric conditions gains paramount significance.
A theoretical background for current signature based eccentricity detection in induction motors has been provided by S. Nandi, R. M. Bharadwaj and H. A. Toliyat in “Mixed eccentricity in three phase induction machines: Analysis, simulation and experiments,” published in Conf. Rec. IEEE-IAS Annual Meeting, Pittsburgh, Pa., vol. 3, pp. 1525-1532, October 2002. A modified winding function approach (MWFA) based modeling of machines reported in this work has been used in the field of condition monitoring for many subsequent researches.
In the field of synchronous machines, an index for non-invasive diagnosis of eccentricity in permanent magnet machines has been proposed by B. M. Ebrahimi, J. Faiz and M. J. Roshtkhari in “Static-eccentricity, dynamic-eccentricity, and mixed-eccentricity fault diagnoses in pelitianent magnet synchronous motors,” published in IEEE Trans. Industrial Electronics, vol. 56, no. 11, pp. 4727-4739, November 2009. As all types of eccentricities affected the same frequency components the authors had to resort to k-Nearest Neighbour classifier and three-layer Artificial Neural Network to detect the type and degree of eccentricity. The network was trained using 280 current vectors out of a total population of 400 current vectors.
C. Bruzzese and G. Joksimovic have identified the harmonic components present in the rotor field and stator voltages of a no-load salient pole synchronous generator in “Harmonic signatures of static eccentricities in the stator voltages and in the rotor current of no-load salient-pole synchronous generators,” published in IEEE Trans. Industrial Electronics, vol. 58, no. 5, pp. 1606-1624, May 2011. The effect of the stator magneto-motive force (MMF) has not been included as the authors have focused their attention on synchronous generator operating at no-load. In case of a synchronous motor, the effect of the stator MMF can play a very significant role in the generation of eccentricity related harmonics.
None of the contemporary works have taken into consideration the effect of supply unbalance, supply harmonics, constructional asymmetry etc. Accordingly, there is a need for a method to determine the specific harmonic components in the motor current spectrum, which will indicate the type and the degree of eccentricity fault under any level of load, supply unbalance and internal asymmetry of the machine.